A metric is the measure you watch. It is not the outcome you are trying to reach by a date. That is an objective. The metrics register keeps the measures in one place so reviews use the same numbers.
What a metric shows
Each metric has a unit, a target, and an owner, and it belongs to a management system that is active for the organisation. Status shows whether it is still being captured, still in use, or retired. The counts show how many metrics are on the register, how many are active, and how many belong to ISO 27001 or ISO 42001 when those systems are selected.
Keeping the measure separate from the objective means a target can be watched over time, and an objective can point at that measure when it needs a result by a due date.
When you need to change it
The metrics register describes each field.